Our objectives are: 1) to learn precisely how the normal human small intestine functions with respect to water and electrolyte transport, and 2) to determine how normal transport processes are deranged in diarrheal diseases. To reach our goal we have developed an accurate method for studying intestinal absorption, secretion, and permeability in normal subjects and in patients with intestinal disease. We have carried out detailed studies to elucidate normal transport processes in the jejunum and ileum and have developed conceptual models which we believe accurately depict the mechanism of water, sodium, potassium, hydrogen, chloride and bicarbonate transfer. We have applied our methods and techniques to the study of patients with celiac sprue, ileostomy diarrhea, congenital chloridorrhea and diabetic diarrhea; studies in uremia, viral gastroenteritis, potassium deficiency and cholera are planned for the future.